I. Field
The present disclosure relates generally to electronics, and more specifically to sigma-delta (ΣΔ) modulation.
II. Background
ΣΔ modulators are widely used for various applications such as oversampling audio digital-to-analog converters (DACs), oversampling analog-to-digital converters (ADCs), instrumentation DACs, and so on. A ΣΔ modulator receives a digital input with many bits (e.g., 16 bits) of resolution at a low input sample rate and generates a digital output having the same resolution but using one or few bits at a high output sample rate. The ΣΔ modulator can generate the digital output using a quantizer having one or few bits of resolution and can thus achieve good linearity. Furthermore, the ΣΔ modulator can spectrally shape quantization noise such that most of the noise is pushed away from the desired signal band toward higher frequencies. The out-of-band noise at higher frequencies can be more easily filtered with a simple analog filter.
However, the high frequency out-of-band noise from the ΣΔ modulator may cause certain problems even in the presence of analog filtering. For example, the out-of-band noise may mix with other signals prior to filtering and fold back into the desired signal band, thus raising the in-band noise floor. The higher noise floor may cause the ΣΔ modulator to fail signal-to-noise ratio (SNR) and/or other specifications. Furthermore, the out-of-band noise may be processed by digital circuitry located on an analog integrated circuit (IC) die. The quantization noise directly translates into the activity rate of the digital circuitry during sensitive periods of operation and may corrupt analog circuit blocks located nearby, thus raising the noise floor of these analog circuit blocks. These deleterious effects due to out-of-band noise from the ΣΔ modulator are undesirable and may even be detrimental.
There is therefore a need in the art for techniques to reduce out-of-band noise from a ΣΔ modulator.